Predictive position feedback controller for web guide control system

ABSTRACT

An electronic controller for a process control system for use with a bi-directional single speed actuating device wherein the controller includes a proportional plus integral mode with predictive position feedback.

- United States Patent 1 Wilson et a1.

[ July 24, 1973 PREDICTIVE POSITION FEEDBACK CONTROLLER FOR WEB GUIDECONTROL SYSTEM [75] Inventors: Francis B. Wilson, Wauconda;

Mathew E. Rodenkirch, Chicago,

both of 111.

[73] Assignee: The Singer-Company, New York,

[22] Filed: Oct. 22, 1971 21 Appl. No.2 191,677

[56] References Cited UNITED STATES PATENTS 3,675,050 7/1972 Paul307/304 3,227,935. 1/1966 Kawada 318/610 2,668,264 2/1954 William, Jr.318/610 X 3,510,737 5/1970 Brown et al. 318/621 X 3,613,419 10/1971Silva 318/621 X Primary Examiner-T. E. Lynch Attorney -.IuTian Falk,Lloyd L. Zickert et a1.

[57] ABSTRACT An electronic controller for a process control system foruse with a bi-directional single speed actuating de vice wherein thecontroller includes a proportional plus integral mode with predictiveposition feedback.

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INVENTORS FRANCIS B. WILSON MATHEW E. RODENKIRCH PREDICTIVE POSITIONFEEDBACK CONTROLLER FOR WEB GUIDE CONTROL SYSTEM This invention relatesin general to a control system for maintaining a process variable withinparameters of a predetermined set point, and more particularly to anelectronic process controller to perform a stabilizing control function,and still more particularly to a controller for proportionallypositioning a manipulated variable in response to an electric inputsignal, where the manipulated variable may be a hydraulic cylinder or areversible electric motor.

Process output is detected by a sensor which generates a signal for acontroller that drives an actuator to control said process. In anintegral or reset mode control, the speed of the manipulated variable isproportional to the error signal. Therefore, in slow processes, such aswhere there is a time lag between the value of the control variable andthe position of the manipulated variable, the integral mode controlusually results in instability. To overcome this problem and accomplishstability, it is common to use proportional mode control or proportionalmode plus integral mode control. An integral mode controller can be madeto function as a proportional mode controller by adding positionfeedback, this usually being in the form of a position transmitterlocated at the control cylinder. Such position feedback arrangementsrequire relatively complex and costly hardware.

The present invention converts the proportional floating or single speedfloating controller to a positioner without the use of a positiontransmitter, thereby eliminating hardware, and at the same time reducingcosts. Moreover, the actuator portion of the controller is isolated fromthe control portion, contrary to previous devices. The invention may beapplied to a wide range of control systems such as non-self regulatingtypes, or where the dynamics indicate the need for proportional plusintegral type control.

The electronic controller of this invention constitutes a proportionalplus integral mode controller with predictive position feedback capableof performing a stabilizing control function in' a process controlsystem which is basically proportional plus integral in nature, but doesnot include actual position feedback. It is useful with a bi-directionalsingle speed actuating device suchas a four-way directional controlvalve-cylinder combination or a reversible single speed electric motor.

It is therefore an object of the present invention to provide a new andimproved electronic controller for a process control system, which iscapable of performing a stabilizing control function.

Another object of this invention is in the provision of a controller fora process control system that includes proportionaland integral modeswith predictive position feedback which eliminates the positive positionfeedback, and thereby reduces the hardware and controls needed toprovide stability.

A still further object of this invention is in the provision of anelectronic controller for process control systems which provides apositioner control without the use of a position transmitter respondingto the control cylinder, and which isolates the actuator from thecontrol portion of the system.

Other objects, features and advantages of the invention will be apparentfrom the following detailed disclosure, taken in conjunction with theaccompanying sheets of drawings, wherein like reference numerals referto like parts, in which:

FIG. 1 is a block diagram showing a conventional process control loopwith true valve position feedback;

FIG. 2 is a combination block and schematic diagram of a process controlloop according to the present invention which includes predictiveposition feedback of the valve;

FIG. 3 is a timing diagram illustrating the proportional action of theactuator in accordance with the process control loop of FIG. 2;

FIG. 4 is a timing diagram illustrating the nature of the output of thecontroller in FIG. 2 with constant process error;

FIG. 5 is a combination diagrammatic and schematic view ofa modifiedprocess control loop illustrating a simpler version than that of FIG. 2wherein the reset action is obtained by discharge action of a resistorin the feedback loop;

FIG. MS a combination diagrammatic and schematic diagram illustrating afurther modification of the invention where the reset action isgenerated by the feedback integrator, but with complete isolation ofstroking time and reset actions;

FIG. 7 is a combination diagrammatic and schematic diagram illustratinga further modification of the invention where the reset action isobtained by discharge action of a resistor in the feedback loop, butwith complete isolation of stroking time and reset actions; and

FIG. 8 is a timing diagram illustrating the overall control action ofthe circuits shown in FIGSJ6 or 7 under constant processerror.

The present invention is best understood by referring to the prior art,as illustrated in FIG. 1, wherein a conventional process control loop isillustrated. The actuator 10 is driven by a conventional valve 11 andtogether comprise a conventionalfour-way directional controlvalve-cylinder combination which will produce a bidirectional singlespeed operation. Alternatively, a reversible electric motor could beprovided, or any other type of: electric signal responsivebi-directional single speed actuatingdevice. The finalcontrol element12, such as a controlvalve, louvre, steering rool, etc. is controlled asto position by the actuator 10 for the processes of the system. At adesired control point, the process is sensed bya suitable sensor whichproduces through a process transmitter 13 a process signal voltage whichis delivered to a summing junction 14. A set point voltage is also fedto the summing junction 14 from which a process error signal (e,,)is-produced and delivered to a proportional'plus integral (P+l) modecontroller 15. The controller 15 amplifies the signal and produces acontroller output voltage (V,,) which is delivered to a second summingjunction 16. The process signal will be the opposite polarity of the setpoint signal and when it differs from the set point signal a processerror signal will be produced out of the summing junction calling forcorrection of the process variable. A feedback voltage (V is producedfrom a suitable device atthe actuator 10 and transmitted by the positiontransmitter 17 to the summing junction 16, it beingappreciated that thefeedback voltage changes as the position of the actuator changes andcoacts with the output voltage (V,,) to produce an error'voltage (e) fordriving the valve 11. The actuator position ((X,) is therefore directlyascertained, and follows the controller'output (V The valve 11 isusually of the on-of type and it is therefore necessary to add a deadzone to stabilize the inner loop. Where the actuator is of anintegrating type, such as a hydraulic cylinder or electric motor, theactuator will move at full speed whenever the error exceeds the deadzone. It is this type of actuator that is utilized in the controller ofthe present invention wherein the change in actuator position can beinferred from the time the output voltage is on.

The controller of the present invention in one embodiment, asillustrated in FIG. 2, differs from the prior art shown in FIG. I inthat the true position of the actuator is not actually sensed and usedfor producing a feedback voltage, but a feedback voltage is producedfrom an electrical integrator driven by the same voltage (V) thatoperates the valve 11. The elements in FIG. 2 that are identical tothose shown in FIG. 1 are identified by the same numerals. The signalvoltage (e) from the summing junction 16 is fed to a Schmitt trigger 20,well known as a hysteretic switching device, which when triggerd appliesvoltage to the valve 11 to drive the actuator at full speed in onedirection. The integrator 21 is driven by the same voltage (V) thatoperates the valve 11 to produce a feedback voltage (V,) of oppositesign to the output voltage (V which starts at zero and increaseslinearly to sum out one hundred percent of V, at the summing junction 16when equal to the full stroking time of the actuator. When V, equalsV,,, the Schmitt trigger 20 will close dropping V to zero and therebystopping the actuator. Accordingly, the output V is timed to be on inproportion to the signal V whereby the position X of the actuator willbe proportional to V The integrator 21 includes a high gain operationalamplifier 22 having inputs 23 and 24 and an output 25. The input 23 isconnected through a variable timing resistor (R to the voltage signal V,while the input 24 is connected to ground. A timing capacitor C isconnected across the common of the input 23 and resistor R, and theoutput 25. The integrator timing is adjusted by varying the value of theresistor R By properly scaling the integrator, the time it takes thefeedback voltage V, to sum out 100%, the output voltage V can be madeequal to the full stroking time of the actuator.

The proportional stroking action of the actuator is shown in FIG. 3where for simplicity sake the triggering level of the Schmitt triggeroccurs at zero level. However, the triggering level of the Schmitttrigger is much smaller than the output voltage V, so that the outputvoltage is available for the timing action of the integrator. Thecontroller integration time is set for the full range of the signal Vequal to the full stroking time of the cylinder.

In FIG. 2, the controller is generally designated by the numeral 30which includes the proportional plus integral mode (PH) and theintegrator 2H. Where constant process error e, is present, the nature ofthe output of the controller is illustrated in FIG. 4. Here it is seenthat the action is a long output pulse V defining proportional actionfollowed by a series of pulses defining reset pulses, the latter ofwhich is produced by the PH mode 15. The on time of the reset pulseswill be a fixed percentage of the full scale stroking time of theactuator, while the time between pulses or the frequency of the pulseswill vary with the signal level. The reset action results in a joggingaction of the actuator.

A similar version ofthe controller according to the invention isillustrated in FIG. 5, wherein the rest action is obtained by adischarge action of a resistor in the integrator. In this version theprocess error signal e is delivered to a proportional amplifier 35 whichmay be a conventional analog amplifier to amplify the process errorsignal and feed it to a summing junction 36. A feedback voltage V, isalso fed to the summing junction 36 and is produced by an integrator 37driven by the same voltage V which goes to the control valve of theactuator. Accordingly, the summing junction 36 compares the amplifierprocess error signal V, and the feedback signal V, to deliver a voltagesignal e to the Schmitt trigger 38. The process error signal e, isderived from the summing junction 39 which compares the process signalwith the set point voltage.

The integrator 37 and the feedback loop includes an operationalamplifier 40 have a capacitor 41 (C and a resistor 42 (R connected inparallel and across the input 43 and the output 44. A resistor 45 (R-,)is is connected to the common of the input 43, the capacitor 41 and theresistor 42. A reset action is obtained from the discharge action ofresistor 42. This controller has the advantage in that the amplifieroutput V,, will not wind-up under constant process error e,,. The valueof resistor 45 is less than the value of resistor 42 to thereby preventinteraction between the stroking time and the reset rate adjustment.

Further versions of the controller according to the invention areillustrated in FIG. 6 and FIG. 7 wherein the reset action is generatedby the feedback integrator (FIG. 6) or by capacitor discharge (FIG. 7),and where complete isolation of the actuator stroking time and resetactions is obtained. Identical elements in this version to those in theversion of FIG. 5 are indicated by the same numerals. Reset pulses canbe generated even if the output of the amplifier 35 (I(,,) is saturated,as is the case with the version of FIG. 5. Therefore, even if thestroking time adjustment is set lower than actual cylinder strokingtime, the reset action will eventually drive the cylinder to therequired position to reduce the error.

The integrator in the version of FIG. 6 designated by the numeral 50includes an operational amplifier 51 having an input 52 connected to thevoltage V through the timing resistor 53 (R The output 54 is connectedto the summing junction 36, and the timing capacitor 55 (C is connectedacross the input 52 and the output 54. A solid state switch 56 is openwhen the voltage V is on or and closed when the voltage equals zero, andtherefore the switch 56 is controlled by the voltage V. When the switch56 is closed an inverter 57 driven by the output of the amplifier 35applies an inverted voltage signal (V to the resistor 58 (R which isconnected in common to the input 52 of the operational amplifier 51, thetiming resistor 53 and the timing capacitor 55. Accordingly, the resetadjustment of resistor 58 will not affect stroking time. This version,like that illustrated in FIG. 5, also avoids reset windup of theamplifier 35 output under constant process error.

The integrator in the version of FIG. 7 designated by the numeral 59includes an operational amplifier 60 having an input 61 connected to thevoltage V through the timing resistor 62 (R The output 63 is connectedto the summing junction 36, and the timing capacitor 64 (C is connectedacross the input 61 and the output 63. Reset resistance 67 (R in serieswith solid state switch 65 are connected across timing capacitor 64 (CSolid state switch 65 is open when voltage V is on" or and closed whenthe voltage V is zero. Switch 65 is gated on or off thru logic circuit66. When the switch 65 is closed capacitor 64 (C is discharged, droppingV until e equals the Schmitt threshold, thereby releasing a reset pulse.Accordingly the reset adjustment of resistor 67 will not affect strokingtime. This version also avoids reset windup of the amplifier 35 outputunder constant process error. The voltage output of the logic circuit, Vis whenever the output V is on or and whenever the output V is zero, andthe switch 65 is closed.

The overall'control action of the versions in FIG. 6 and FIG. 7 underconstant process error is shown in FIG. 8. While the final outputactions of the voltage V and the position of the actuator X areidentical to that of the version of FIG. 2, as shown in FIG. 4, thereset pulses are generated by driving the feedback voltage V, back downuntil the voltage signal e to the Schmitt trigger equals the onthreshold voltage e,. During this action, the output voltage signal(V,,) of the amplifier 35 remains constant and equal to the constantprocess error voltage (e,,k,,) and the feedback voltage V, variesbetween V, (output off) and V, plus the threshold voltage (2,) (outputon) of the Schmitt trigger. When the output voltage V comes on it drivesthe feedback voltage V, toward minus V," at the maximum rate pursuant tothe formula V,= V/R C x t. When the output is off, the reset actiondrives the feedback voltage V, down until the error voltage (e) equalsthe threshold voltage (e,) of the Schmitt trigger and at the rate V,=VIR C x t. The variable voltage output V, of the proportional amplifier35 is such that the off time of the Schmitt trigger varies in relationto this output level. Therefore, the greater the output voltage V,,, thegreater will be the frequency of the reset pulses and the overallcontrol action will approximate that of a true proportional plusintegral controller.

The final output voltage V to the control valve is three-state on-o sothat typical output state conditions would be a full positive voltagewhere the cylinder is moving in one direction, a full negative voltagewhere the cylinder is moving in the opposite direction, and a zerovoltage where the cylinder is stopped. Further, the output of theproportional amplifier 35 is a bidirectional (i) proportional signal toact in accordance with the process error signal (t). The integratorfeedback voltage V, is likewise i and always opposite in sign to theoutput voltage signal V, of the proportional amplifier. During theproportional pulsing the integrator voltage V, sums out the change inthe proportional amplifier V5. The value of the timing resistor 53 (Rcan be adjusted to vary this rate to match cylinder speeds. By properlyscaling the timing resistor 53, the time required to sum out 100% of theamplifier voltage output can be equal to the time it takes the cylinderto go full stroke. Therefore, the action of the controller is that of atrue positioner wherein 100% change in the output voltage results in100% change in cylinder position, or 50% change in output voltageresults in 50% change in cylinder position, etc.

Accordingly, the controller where it provides a predictive positionfeedback defines a proportional plus integral control action using anon-off valve or electric motor without the need of a positiontransmitter.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention, but it is understood that this application is to be limitedonly by the scope of the appended claims.

The invention is hereby claimed as follows:

1. In an apparatus for process control including an electric signalresponsive bidirectional single speed actuating device for controllingthe position of a manipulated variable, means for sensing the process ata control point and for producing a process signal, and an electroniccontroller receiving said signal and controlling a timed voltage fordriving the actuating device, the improvement being in the controllerwhich comprises amplifying means receiving an error signal produced bycomparing the process signal and a set point signal and amplifying sameto produce an amplified output signal, a hysteretic switching devicebetween the amplifying means and the actuating device triggered by theamplified output signal and controlling a voltage for the actuatingdevice, and an integrator producing a feedback signal opposite-to saidoutput signal and having its input connected to the input voltage to theactuating device and its output summed with said output signal toproduce a summed signal to time the summed signal to be on in proportionto the output signal wherein the position of the actuating device willbe proportional to the output signal.

2. The controller of claim 1, wherein the integrator includes anoperational amplifier having an input connected to the actuating devicevoltage and an output connected to a summing junction with the amplifiedoutput signal, a timing capacitor connected across said output and saidinput of the operational amplifier, and a timing resistor connectedbetween said input and the actuating device voltage, wherein adjustmentof the resistor adjusts the timing of the integrator.

3. The controller of claim 2, wherein the resistor is variable to adjustthe timing of the integrator.

4. The controller of claim 1, and means for providing reset action inresponse to constant process error.

5. The controller of claim 4, wherein said reset action means includesan integral mode combined withsaid proportional amplifier. I

6. The controller of claim 2, and means for providing reset action inresponse to constant process error.

7. The controller .of claim 6, wherein said reset action means includesa resistor connected in the integrator in parallel with the timingcapacitor which discharges to provide the reset action.

8. The controller of claim 6, wherein said reset action means includes areset circuit consisting of a resistor connected inthe integrator inseries with an inverter between the amplified output signal and theinput of the operational amplifier, and switch means responsive to theactuating device voltage to turn the circuit off when the actuatingdevice voltage is on and to turn the circuit on when the actuatingdevice voltage is off.

9. The controller of claim 6, wherein said reset action means includes areset circuit consisting of a series combination of a resistor and asolid state switch connected in the integrator in parallel with thetiming capacitor, said switch responsive to the actuating device voltageto turn the circuit off when the actuating device voltage is on and toturn the circuit on when the actuating device voltage is off.

10. In an apparatus for controlling a process variable including anelectric signal responsive bi-directional single speed actuating devicefor controlling a manipulated variable, means for sensing the process ata control point and for producing a process signal, and an electroniccontroller receiving said signal and controlling a timed voltage fordriving the actuating device, the improvement being in the controllerwhich comprises a proportional plus integral mode receiving an errorsignal produced by comparing the process signal and a set point signaland amplifying same to produce an amplified output signal and producingreset action, a hysteretic switching device between the proportionalplus integralmode and the actuating device triggered by the amplifiedoutput signal and controlling a voltage for the actuating, device,and'an integrator producing a feedback signal opposite to said outputsignal and having its input connected to the input voltage to theactuating device and its output summed with said output signal toproduce a summed signal to time the summed signal to be on in proportionto the output signal wherein the position of the actuating device willbe proportional to the output signal.

1 l. The controller of claim 10, wherein the integrator includes anoperational amplifier having an input connected to the actuating devicevoltage and an output connected to a summing junction with the amplifiedoutput signal, a timing capacitor connected across the output and theinput of the operational amplifier, and a timing resistor connectedbetween the operational amplifier input and the actuating devicevoltage, wherein adjustment of the resistor adjusts the timing of theintegrator.

12. The controller of claim 11, wherein the resistor is variable toadjust the timing of the integrator.

13. In an apparatus for process control including an electric signalresponsive bi-directional single speed actuating device for controllingthe position of a manipulated variable, means for sensing the process ata control point and for producing a process signal, means for producinga set point signal, and an electronic controller receiving and comparingthe process and set point signals to produce an error signal whichcontrols a timed voltage for driving the actuating device, theimprovement being in the controller which comprises a hystereticswitching device receiving and triggered by the error signal andcontrolling a driving voltage for the actuating device, and anintegrator producing a feedback signal opposite to said error signalhaving its input connected to the driving voltage of the actuatingdevice and its output summed with the error signal to produce a summedsignal to maintain said driving voltage on in proportion to said errorsignal, wherein the position of signal.

1. In an apparatus for process control including an electric signalresponsive bidirectional single speed actuating device for controllingthe position of a manipulated variable, means for sensing the process ata control point and for producing a process signal, and an electroniccontroller receiving said signal and controlling a timed voltage fordriving the actuating device, the improvement being in the controllerwhich comprises amplifying means receiving an error signal produced bycomparing the process signal and a set point signal and amplifying sameto produce an amplified output signal, a hysteretic switching devicebetween the amplifying means and the actuating device triggered by theamplified output signal and controlling a voltage for the actuatingdevice, and an integrator producing a feedback signal opposite to saidoutput signal and having its input connected to the input voltage to theactuating device and its output summed with said output signal toproduce a summed signal to time the summed signal to be on in proportionto the output signal wherein the position of the actuating device willbe proportional to the output signal.
 2. The controller of claim 1,wherein the integrator includes an operational amplifier having an inputconnected to the actuating device voltage and an output connected to asumming junction with the amplified output signal, a timing capacitorconnected across said output and said input of the operationalamplifier, and a timing resistor connected between said input and theactuating device voltage, wherein adjustment of the resistor adjusts thetiming of the integrator.
 3. The controller of claim 2, wherein theresistor is variable to adjust the timing of the integrator.
 4. Thecontroller of claim 1, and means for providing reset action in responseto constant process error.
 5. The controller of claim 4, wherein saidreset action means includes an integral mode combined with saidproportional amplifier.
 6. The controller of claim 2, and means forproviding reset action in response to constant process error.
 7. Thecontroller of claim 6, wherein said reset action means includes aresistor connected in the integrator in parallel with the timingcapacitor which discharges to provide the reset action.
 8. Thecontroller of claim 6, wherein said reset action means includes a resetcircuit consisting of a resistor connected in the integrator in serieswith an inverter between the amplified output signal and the input ofthe operational amplifier, and switch means responsive to the actuatingdevice voltage to turn the circuit off when the actuating device voltageis on and to turn the circuit on when the actuating device voltage isoff.
 9. The controller of claim 6, wherein said reset action meansincludes a reset circuit consisting of a series combination of aresistor and a solid state switch connected in the integrator inparallel with the timing capacitor, said switch responsive to theactuating device voltage to turn the circuit off when the actuatingdevice voltage is on and to turn the circuit on when the actuatingdevice voltage is off.
 10. In an apparatus for controlling a processvariable including an electric signal responsive bi-directional singlespeed actuating device for controlling a manipulated variable, means forsensing the process at a control point and for producing a processsignal, and an electronic controller receiving said signal andcontrolling a timed voltage for driving the actuating device, theimprovement being in the controller which comprises a proportional plusintegral mode receiving an error signal produced by comparing theprocess signal and a set point signal and amplifying same to produce anamplified output signal and producing reset action, a hystereticswitching device between the proportional plus integral mode and theactuating device triggered by the amplified output signal andcontrolling a voltage for the actuating, device, and an integratorproducing a feedback signal opposite to said output signal and havingits input connected to the input voltage to the actuating device and itsoutput summed with said output signal to produce a summed signal to timethe summed signal to be on in proportion to the output signal whereinthe position of the actuating device will be proportional to the outputsignal.
 11. The controller of claim 10, wherein the integrator includesan operational ampliFier having an input connected to the actuatingdevice voltage and an output connected to a summing junction with theamplified output signal, a timing capacitor connected across the outputand the input of the operational amplifier, and a timing resistorconnected between the operational amplifier input and the actuatingdevice voltage, wherein adjustment of the resistor adjusts the timing ofthe integrator.
 12. The controller of claim 11, wherein the resistor isvariable to adjust the timing of the integrator.
 13. In an apparatus forprocess control including an electric signal responsive bi-directionalsingle speed actuating device for controlling the position of amanipulated variable, means for sensing the process at a control pointand for producing a process signal, means for producing a set pointsignal, and an electronic controller receiving and comparing the processand set point signals to produce an error signal which controls a timedvoltage for driving the actuating device, the improvement being in thecontroller which comprises a hysteretic switching device receiving andtriggered by the error signal and controlling a driving voltage for theactuating device, and an integrator producing a feedback signal oppositeto said error signal having its input connected to the driving voltageof the actuating device and its output summed with the error signal toproduce a summed signal to maintain said driving voltage on inproportion to said error signal, wherein the position of the actuatingdevice will be proportional to said error signal.